A reflector sight is an optical device allowing a shooter to look through a glass element and see a reflection of an illuminated aiming point or image that is superimposed on the field of view. A common design is a red-dot sight using a red Light-Emitting Diode (LED) at the focus of collimating optics. The LED generates an illuminated reticle in the form of a dot. The dot indicates a sightline that is in alignment with the weapon the sight is attached to. This way the reflector sight is close to parallax free and insensitive to eye position. Another example is a holographic diffraction sight that provides a holographic reticle image on glass window that is superimposed at a distance on the field of view.
Reflector sights generally fall into two categories, closed sights and open sights. The closed sights typically have a housing of a roughly tubular shape containing the optics. The ends of the closed sights are typically closed by transparent windows preventing direct access to the optics. Dust covers can typically be fitted at the ends. Different filters, such as polarizing filters, can be fitted on the closed sight. Open sights typically has a flat base and a single supporting loop extending from the base and holding the reflective optics. This type of reflector sights does not have a housing protecting the optics from direct access.
Most reflector sights have an adjustable brightness of the reticle. The adjustment can be passive or active.
It is common that Light Machine Guns (LMG), General-Purpose Machine guns (GPMG), and Heavy Machine Guns (HMG) have barrels with quick-change capability. The barrel is typically mounted in a trunnion, which in turn is mounted in a receiver. A hand-operated lever or the like typically unlocks the barrel and the shooter can push it forward and remove it from the trunnion. A carrying handle is typically fixed to the barrel making it easier to handle when changing the barrel. It is not uncommon that trained soldiers can change the barrel in less than 10 seconds, or even in less than 7 seconds. Examples of a LMG, a GPMG, and a HMG that are configured for a quick barrel change is the FN Minimi, the FN MAG, and Browning M2A1.
Different barrels mounted in the same firearm may produce different groupings when shooting at the same distance and with ammunition of the same make and type. The mean positions or geometric center of the different groupings may be shifted horizontally and/or vertically with respect to one another. This means that different barrels produce different groupings, which typically has a negative impact on the accuracy of the weapon system as a whole. For example, if a firearm has two interchangeable barrels configured for a quick barrel change, a shooter may adjust the sights such that the sight line falls between the geometric centers of the two different groupings produced by the two barrels. This can have a severe impact on the accuracy when shooting. Alternatively, the shooter could reset the sights after each barrel change. However, this takes time and involves test shooting, which typically requires a shooting range.